A spokesman said: “Blair Jenkins has called for an honest debate, yet his organisation appears to be guilty of deep dishonesty and deception. Perhaps one way of ensuring an honest debate would be if Blair Jenkins stepped down and removed himself from it.”

The cash was paid to Dr Elliot Bulmer and the piece published in a Scottish newspaper with no indication of the Yes Scotland link.

This matter was first brought to our attention last Wednesday. We responded, confirming a small fee had been paid to Dr Bulmer at his request. We were perfectly relaxed and transparent about this.

A Yes Scotland statement

The revelation comes after police began a probe into email hacking claims by Yes Scotland.

A Yes Scotland statement said: “This matter was first brought to our attention last Wednesday. We responded, confirming a small fee had been paid to Dr Bulmer at his request. We were perfectly relaxed and transparent about this.

“However, it became apparent an email account had been accessed illegally and information relating to this had been gleaned.”

The group said Dr Bulmer was asked if he would write an article and he was paid a nominal fee. But it was stressed: “ We had no input to, or influence over, what he wrote.”

Blair McDougall, of Better Together, said: “How can we believe what they say if they are secretly paying supposedly impartial experts?”